Yes, from what I know, load plan logs get purged implicitly even though no purging process is scheduled.
By default, they get purged after 7 days - this is specified in "Keep Log History" attribute of generated load plan.
To avoid this, after creating the load plan and before executing it: Go to Load Plan and scenarios -> Open the generated load plan -> In Definition tab set Keep Log History (days) and change the value to any higher value, say 30 days and ODI will retain these logs for 30 days.
Hope that helps.
Thanx a lot for the quick answer.
For sure this will be helpful.